Role of sterol biosynthesis key enzyme SQS1 in plant development
Zou Chunxue¹, Li Na¹, Yang Xiao¹, Zhu Hui¹, Li Panpan¹, Kuang Susu¹ and Men Shuzhen^1, *

¹Department of Plant Biology and Ecology, College of Life Sciences, Nankai University, 94 Weijin Road, Nankai District,Tianjin, 300071, P.R. China.
^* Corresponding author. E-mail shuzhenmen@nankai.edu.cn; Tel/Fax +86-22-23500856

Abstract 
Sterols are membrane lipids crucial for cellular functions and development of diverse organisms. SQUALENE SYNTHASE (SQS) catalyzes the first committed step of sterol biosynthesis. In Arabidopsis there are two annotated SQS-coding genomic sequences, SQS1 and SQS2, organized in a tandem array. Previous reports showed that SQS2 has no SQS activity, SQS1 is the only functional SQS in Arabidopsis. However, the function of SQS1 in plant development has not been characterized. We have here isolated two mutant alleles of the SQS1 gene, sqs1-1 and sqs1-2. Both mutants showed embryo lethality phenotype, and the null mutant embryos were arrested at 1 cell stage to early globular stages. Cell division was also aberrant both in the suspensor and embryo proper. To address expression pattern and subcellular localization of SQS1, we made fusion constructions of SQS1 with the reporter genes GFP and GUS, and tested their functionality by complementation of the sqs1-1 embryonic phenotype. SQS1-EGFP and SQS1-GUS expression was detected in the embryo throughout the embryo development, the root meristem and at higher levels in elongating cells, and the vascular tissues. Fluorescence colocalization studies revealed that SQS1 is localized to the ER. We next analyzed localization of PIN1, PIN4 and PIN7 auxin transporter proteins in sqs1-1 embryos. The results showed that in contrast to wild-type, where PIN1 localized in the provascular cells facing the basal embryopole—the hypophysis and to the epidermal cells on the apical membrane, PIN1 signal was weak, apolar and more cytosolic in sqs1-1 mutant embryo cells. To investigate the function of SQS1 in postembryonic development, we generated SQS1 RNAi lines. The SQS1 RNAi plants displayed a short inflorescence stem phenotype with more branches, curly leaves with necrotic spots, which implies a role for SQS1 in plant defense.